/Engine/Plugins/Runtime/MeshModelingToolset/Source/ModelingComponents/Public/Mechanics/CubeGrid.h
Direction that a face is facing. Values are set up like in IndexUtil, such that abs(value)-1 is the nonzero normal direction, and sign(value) is the sign of the normal.
enum EFaceDirection { NegativeX = -1, PositiveX = 1, NegativeY = -2, PositiveY = 2, NegativeZ = -3, PositiveZ = 3, }